The vasculature is recognized as a dynamic metabolic organ that exists under normal physiological conditions in an intact, undisturbed state (Karsan, et al. In: Hematology: Basic Principles and Practice, 3rd Ed. Hoffman, et al. (eds) 2000; pp. 1770-82). Endothelial cells (EC), which line the exposed (luminal) surface of blood vessels, are normally not thrombogenic. That is, healthy EC do not attract or bind circulating platelets (Cines, et al. Blood 1998, 91: 3527-61; May, et al. Thromb Haemost 1999, 82: 962-70). It is well known that the physiological function of the endothelium is to facilitate blood flow by providing a highly thromboresistant surface to flowing blood that inhibits platelet adhesion and clotting (Cines, et al.). However, under inflammatory conditions, the nonthrombotic surface of EC can be transformed to a prothrombotic surface following exposure to cytokines (May, et al.; Diquelou et al. Thromb Haemost 1995, 74: 778-83), resulting in procoagulant activity and a predisposition to thrombosis (May, et al.; Dardik, et al. Br J. Haematol. 2000, 109:512-8; Andre, et al. Blood 2000, 96:3322-8). Indeed, the adhesion, accumulation and recruitment of non-stimulated platelets on cytokine-stimulated EC have been reported, with studies implicating the Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1; Rosenblum, et al. Stroke 1995, 27:709-11); beta 1 integrin (Bombeli et al. J Exp. Med. 1998, 187:329-39), von Willebrand factor (Dardik, et al.; Andre, et al.), and tissue factor (Verheul, et al. Blood 2000, 96:4216-21) in these processes. Thus, under inflammatory conditions, cytokines induce alterations in EC which result in the adhesion of non-stimulated platelets.
Recently, a novel adhesion protein of the immunoglobulin (Ig) superfamily has been described with properties indicating a potential triggering role in the pathogenesis of inflammatory thrombosis, atherosclerosis and other disorders involving thrombosis formation. This protein was identified first on the surface of human platelets and called the F11 receptor (F11R; Kornecki, et al. J Biol Chem 1990, 265: 10042-8; Naik, et al. Biochem J 1995, 311: 155-62), and then on the surface of murine endothelial and epithelial cells and called JAM/JAM-A (Martin-Padura, et al. J. Cell Biol. 1998, 142:117-27).
The human platelet F11 receptor (F11R) is a surface glycoprotein duplex (32 and 35 kD at core protein: 29 kDa) member of the immunoglobulin superfamily. The F11R was first discovered as the target of a potent stimulatory monoclonal antibody, M.Ab.F11, that induces platelet secretion followed by aggregation (Kornecki, et al.; Naik, et al.; Kornecki, et al. J Lab Clin. Med. 1988, 111:618-26; Wang et al. Biochem. J. 1995, 311: 401-6; Kornecki, et al. In: Leukocyte Typing V. Schlossman, et al. (eds.) Oxford University Press 1195: 1241-3; Sobocka, et al. Blood 1997, 90: 10, Supplement 1, Part 2, Nov. 15, 2996a; Sobocka, Ph.D. Thesis, 1998: SUNY Downstate, Brooklyn, N.Y., Presented Jun. 10, 1998; published Sep. 15, 1998; Sobocka, et al. Blood 2000, 95:2600-9; Babinska, et al. Thromb Haemost 2002, 87: 712-21). Signal transduction mechanisms for platelet secretion and aggregation induced by M.Ab.F11 following its initial binding to F11R include: crosslinking of the F11R to the FcγRII (Naik, et al.), activation and translocation of specific PKC isozymes (Wang, et al.), phosphorylation of the F11R through activation of PKC (Naik, et al.; Wang, et al.), phosphorylation of the F11R following induction of platelet aggregation by the physiological agonists thrombin and collagen and by M.Ab.F11 itself (Sobocka, et al. 1997; Sobocka; Sobocka, et al. 2000; Babinska, et al.), and phosphorylation of myosin light chain and pleckstrin, leading to shape change and granular secretion respectively (Kornecki, et al. 1990). Following secretion, this signal transduction pathway culminates in the activation of latent fibrinogen receptors and platelet aggregation (Kornecki, et al. 1990). Partial amino acid sequences representing 30% of the length of purified F11R have been reported by Kornecki in 1995 (Naik, et al.). Cloning of the full-length cDNA for the platelet F11R has revealed that it is a cell adhesion molecule (CAM), a member of the immunoglobulin superfamily (Sobocka, et al. 1997; Sobocka; Sobocka, et al. 2000). As a CAM, the F11R participates in mechanisms underlying adhesion of platelets, endothelial cells, and epithelial cells (Martin-Padura, et al.; Sobocka, et al. 2000).
The conclusion that, in addition to its role as a receptor that triggers signal transduction leading to secretion, the F11R also serves as a CAM involved in platelet adhesion was supported by the high degree of sequence similarity found between the human platelet F11R and an adhesion protein called Junctional Adhesion Molecule (JAM), a protein found in murine endothelial cells (Martin-Padura, et al. 1998). Comparison of the murine JAM sequence to the previously-reported sequences of the human platelet F11R (Naik, et al.) revealed over 70% homology of JAM to the N-terminus (23 amino acids) of F11R and to two enzyme-digested products of F11R. In addition, both the human platelet F11R core protein and the murine JAM protein were found to contain a single transmembrane domain and two pairs of cysteine residues in their extracellular domains that allow formation of intermolecular disulfide bridges forming characteristic Ig-like folds. It is now well established that the protein referred to as JAM (Martin-Padura, et al, 1998; Ozaki, et al. J. Immunol. 1999, 163: 553-7; Williams, et al. Mol. Immunol. 1999, 36: 1175-88; Liu, et al. J. Cell Science 2000, 113: 2363-74; Gupta, et al. IUBMB Life 2000, 50: 51-6; Naik, et al. J. Cell Science 2001, 114: 539-47), is the murine ortholog of the human F11R (Kornecki, et al. 1990; Naik, et al 1995; Kornecki, et al. 1988; Wang, et al.; Kornecki, et al 1995; Sobocka, et al. 1997; Sobocka; Sobocka, et al. 2000; Babinska, et al.). JAM was localized at intercellular junctions of mouse endothelial and epithelial cells (Martin-Padura, et al.). Similarly, the platelet antibody M.Ab.F11 was found to recognize F11R molecules present at intercellular junctions of cultured human umbilical vein endothelial cells (Sobocka, et al. XVIII ISTH Congress, July, 2001, Paris, France, Abs#P1902; Babinska et al., manuscript submitted, 2005). A recent study conducted by the inventors (Babinska, et al. 2002) has determined that two domains of F11R are critical for the induction of platelet aggregation by M.Ab.F11 and the adhesion of platelets to M.Ab.F11. Heretofore, the role of F11R in physiological and pathophysiological processes involving the adhesion of platelets to cytokine-inflamed endothelial cells has remained unknown. The inventors have now determined that the N-terminus of F11R and the first Ig fold of F11R contain protein sequences which are critical for the adhesion of platelets to endothelial cells, and that the recombinant soluble F11R protein and F11R-peptides block approximately 60% of the force of adhesion of platelets to cytokine-treated EC, demonstrating the involvement of the F11R protein in platelet-endothelial cell interactions, which under pathological conditions, result in thrombosis, atherosclerosis and other disorders involving thrombosis formation.